A die for extruding a tubing typically comprises three parts: an outer tube or housing, a central mandrel defining within the tube an annular flow chamber, and a plurality of webs extending between the outer tube and the inner mandrel and supporting this mandrel rigidly within the housing. The housing and mandrel are both centered on a die axis and the webs extend radially of this die axis. The webs further subdivide the annular flow chamber into a plurality of angularly spaced and axially extending flow passages.
Plastified synthetic-resin material forced axially into the upstream end of the die is formed into an annular body that is subdivided into a plurality of angularly spaced and axially flowing strands by the webs which support the mandrel in the housing. These strands, still in molten condition after passing the webs, reunite downstream of the webs and form an integral and seamless tube that is extruded from the downstream end of the die, usually into a calibrating sleeve that establishes the outside diameter of the tube as the resin hardens.
In small-diameter tubing the interruptions caused by the webs are almost imperceptible. As the tubing diameter and thickness increases, however, the webs leave axially extending grooves often up to a millimeter or more deep in the tubing. Not only do these grooves give the tubing an unattractive appearance, but they substantially weaken the tubing so that it is less able to withstand internal pressure than would otherwise be the case. Not only does the decrease in wall thickness weaken the tubing, but the webs orient the relatively long synthetic-resin molecules parallel to the axis of the tubing so that the resin itself at these grooves is oriented longitudinally of the tubing and has little transverse strength.
Several attempts have been made to overcome this disadvantage. The most obvious method is simply to lengthen the die considerably, keeping the resin molten for a considerable period of time after it flows around the webs so as to insure its complete homogenization before exiting from the downstream end of the die. Such a construction is very expensive and requires extremely close tolerances, since the support for the mandrel is spaced relatively far from the downstream end of this mandrel. It has also been suggested to form the outside of the mandrel and/or the inside of the housing with helically extending grooves downstream of the webs to insure that the synthetic-resin is mixed and homogenized. In another system the webs are made to extend not strictly axially, but in directions inclined to the axis so as themselves to act as small mixers and homogenize the resin. All of these arrangements have the disadvantage that they required a relatively complicated and, hence, expensive construction of the tube-forming die.